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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | VERSIONS | ATTRIBUTES | CONFORMING TO | NOTES | BUGS | SEE ALSO | COLOPHON |
FENV(3) Linux Programmer's Manual FENV(3)
feclearexcept, fegetexceptflag, feraiseexcept, fesetexceptflag,
fetestexcept, fegetenv, fegetround, feholdexcept, fesetround, fes‐
etenv, feupdateenv, feenableexcept, fedisableexcept, fegetexcept -
floating-point rounding and exception handling
#include <fenv.h>
int feclearexcept(int excepts);
int fegetexceptflag(fexcept_t *flagp, int excepts);
int feraiseexcept(int excepts);
int fesetexceptflag(const fexcept_t *flagp, int excepts);
int fetestexcept(int excepts);
int fegetround(void);
int fesetround(int rounding_mode);
int fegetenv(fenv_t *envp);
int feholdexcept(fenv_t *envp);
int fesetenv(const fenv_t *envp);
int feupdateenv(const fenv_t *envp);
Link with -lm.
These eleven functions were defined in C99, and describe the handling
of floating-point rounding and exceptions (overflow, zero-divide,
etc.).
Exceptions
The divide-by-zero exception occurs when an operation on finite
numbers produces infinity as exact answer.
The overflow exception occurs when a result has to be represented as
a floating-point number, but has (much) larger absolute value than
the largest (finite) floating-point number that is representable.
The underflow exception occurs when a result has to be represented as
a floating-point number, but has smaller absolute value than the
smallest positive normalized floating-point number (and would lose
much accuracy when represented as a denormalized number).
The inexact exception occurs when the rounded result of an operation
is not equal to the infinite precision result. It may occur whenever
overflow or underflow occurs.
The invalid exception occurs when there is no well-defined result for
an operation, as for 0/0 or infinity - infinity or sqrt(-1).
Exception handling
Exceptions are represented in two ways: as a single bit (exception
present/absent), and these bits correspond in some implementation-
defined way with bit positions in an integer, and also as an opaque
structure that may contain more information about the exception
(perhaps the code address where it occurred).
Each of the macros FE_DIVBYZERO, FE_INEXACT, FE_INVALID, FE_OVERFLOW,
FE_UNDERFLOW is defined when the implementation supports handling of
the corresponding exception, and if so then defines the corresponding
bit(s), so that one can call exception handling functions, for
example, using the integer argument FE_OVERFLOW|FE_UNDERFLOW. Other
exceptions may be supported. The macro FE_ALL_EXCEPT is the bitwise
OR of all bits corresponding to supported exceptions.
The feclearexcept() function clears the supported exceptions
represented by the bits in its argument.
The fegetexceptflag() function stores a representation of the state
of the exception flags represented by the argument excepts in the
opaque object *flagp.
The feraiseexcept() function raises the supported exceptions
represented by the bits in excepts.
The fesetexceptflag() function sets the complete status for the
exceptions represented by excepts to the value *flagp. This value
must have been obtained by an earlier call of fegetexceptflag() with
a last argument that contained all bits in excepts.
The fetestexcept() function returns a word in which the bits are set
that were set in the argument excepts and for which the corresponding
exception is currently set.
Rounding mode
The rounding mode determines how the result of floating-point
operations is treated when the result cannot be exactly represented
in the significand. Various rounding modes may be provided: round to
nearest (the default), round up (toward positive infinity), round
down (toward negative infinity), and round toward zero.
Each of the macros FE_TONEAREST, FE_UPWARD, FE_DOWNWARD, and
FE_TOWARDZERO is defined when the implementation supports getting and
setting the corresponding rounding direction.
The fegetround() function returns the macro corresponding to the
current rounding mode.
The fesetround() function sets the rounding mode as specified by its
argument and returns zero when it was successful.
C99 and POSIX.1-2008 specify an identifier, FLT_ROUNDS, defined in
<float.h>, which indicates the implementation-defined rounding
behavior for floating-point addition. This identifier has one of the
following values:
-1 The rounding mode is not determinable.
0 Rounding is toward 0.
1 Rounding is toward nearest number.
2 Rounding is toward positive infinity.
3 Rounding is toward negative infinity.
Other values represent machine-dependent, nonstandard rounding modes.
The value of FLT_ROUNDS should reflect the current rounding mode as
set by fesetround() (but see BUGS).
Floating-point environment
The entire floating-point environment, including control modes and
status flags, can be handled as one opaque object, of type fenv_t.
The default environment is denoted by FE_DFL_ENV (of type const
fenv_t *). This is the environment setup at program start and it is
defined by ISO C to have round to nearest, all exceptions cleared and
a nonstop (continue on exceptions) mode.
The fegetenv() function saves the current floating-point environment
in the object *envp.
The feholdexcept() function does the same, then clears all exception
flags, and sets a nonstop (continue on exceptions) mode, if
available. It returns zero when successful.
The fesetenv() function restores the floating-point environment from
the object *envp. This object must be known to be valid, for
example, the result of a call to fegetenv() or feholdexcept() or
equal to FE_DFL_ENV. This call does not raise exceptions.
The feupdateenv() function installs the floating-point environment
represented by the object *envp, except that currently raised
exceptions are not cleared. After calling this function, the raised
exceptions will be a bitwise OR of those previously set with those in
*envp. As before, the object *envp must be known to be valid.
These functions return zero on success and nonzero if an error
occurred.
These functions first appeared in glibc in version 2.1.
For an explanation of the terms used in this section, see
attributes(7).
┌────────────────────────────────────┬───────────────┬─────────┐
│Interface │ Attribute │ Value │
├────────────────────────────────────┼───────────────┼─────────┤
│feclearexcept(), fegetexceptflag(), │ Thread safety │ MT-Safe │
│feraiseexcept(), fesetexceptflag(), │ │ │
│fetestexcept(), fegetround(), │ │ │
│fesetround(), fegetenv(), │ │ │
│feholdexcept(), fesetenv(), │ │ │
│feupdateenv(), feenableexcept(), │ │ │
│fedisableexcept(), fegetexcept() │ │ │
└────────────────────────────────────┴───────────────┴─────────┘
IEC 60559 (IEC 559:1989), ANSI/IEEE 854, C99, POSIX.1-2001.
Glibc notes
If possible, the GNU C Library defines a macro FE_NOMASK_ENV which
represents an environment where every exception raised causes a trap
to occur. You can test for this macro using #ifdef. It is defined
only if _GNU_SOURCE is defined. The C99 standard does not define a
way to set individual bits in the floating-point mask, for example,
to trap on specific flags. Since version 2.2, glibc supports the
functions feenableexcept() and fedisableexcept() to set individual
floating-point traps, and fegetexcept() to query the state.
#define _GNU_SOURCE /* See feature_test_macros(7) */
#include <fenv.h>
int feenableexcept(int excepts);
int fedisableexcept(int excepts);
int fegetexcept(void);
The feenableexcept() and fedisableexcept() functions enable (disable)
traps for each of the exceptions represented by excepts and return
the previous set of enabled exceptions when successful, and -1
otherwise. The fegetexcept() function returns the set of all
currently enabled exceptions.
C99 specifies that the value of FLT_ROUNDS should reflect changes to
the current rounding mode, as set by fesetround(). Currently, this
does not occur: FLT_ROUNDS always has the value 1.
math_error(7)
This page is part of release 4.12 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
Linux 2015-03-02 FENV(3)
Pages that refer to this page: execve(2), fma(3), j0(3), lrint(3), lround(3), matherr(3), pthread_create(3), remainder(3), rint(3), round(3), __setfpucw(3), y0(3), math_error(7)